A fundamental requirement for beam delivery configurations in medical laser systems is an accurately predictable beam profile at the treatment site, eg. an anterior or internal corneal treatment surface. Known delivery systems generally include, among other components, a beam shaping means, typically defining an aperture that sets the beam cross-section, a scanner, and a fluence control. The latter is set to control fluence—energy density at a cross-section—to a fixed figure or at least below a predetermined limit for a given instrument and/or procedure. Such limits are usually predetermined by regulatory authorities and adherence to them is generally a mandatory condition of marketing approval by such authorities.
International patent publication WO 98/57604 discloses a laser beam delivery procedure and configuration in which the beam cross-section is varied during scanning, typically by being progressively increased as the surface being ablated is scanned in a predetermined pattern. Suitable scanning apparatus for this purpose is described in international patent publication WO 98/04303: that system has the particular benefit of maintaining the beam collimated and parallel to a fixed direction as it is laterally scanned. However, perfect collimation is often disturbed by at least two effects. Firstly, there is the disturbance arising from fluence control downstream of the scanner. Secondly, many laser device beam outputs are significantly variable in their cross-section and energy profile, and this causes still further variations at the fluence control as the latter in turn compensates for beam fluctuations.
The overall result of these imperfections is that, while a laser bream delivery system may incorporate a scanner arrangement in which the beam theoretically remains optically collimated as it is scanned, the system is still essentially height or z-axis sensitive, ie. the actual beam cross-section is dependent to some degree on the exact location of the treatment surface on the optical path or axis. In consequence, a predictable outcome of the procedure is dependent on having the treatment surface at an accurate location and on taking steps to prevent z-axis movement of the surface during the procedure. This adds to the complexity and sensitivity of typical ophthalmic laser surgery treatments: it would be preferable to provide an optical beam delivery system in which height or z-axis sensitivity was reduced, while making allowance for the reality of beam fluctuations in laser device outputs.